Electrochemical polymerization of β-substituted and β,β'-disubstituted selenophenes

225

J. Electroanal. Chem., 238 (1987) 225-237 Elsevier Sequoia !%A., Lausanne - Printed in The Netherlands

ELECTROCHEMICAL /3,/l’-DISUBSTITUTED

GERARD

DIAN

POLYMERIZATION SELENOPHENES

l, NADINE

MERLET

and GERARD

OF /3-SUBSTITUTED

AND

BARBEY

Laboratorre de Chimie Analytique et d’Electrochrmie Organrque, II. E. R. Scrences, llnrversit6 de Haute-Normandre, 76130 Mont-Saint-Algnan (France) FRANCIS

OUTURQUIN

Laboratorre de Synthbe 76130 Mont-Saint-Algnan

and CLAUDE

PAULMIER

de ComposPs OrganostGmk, (France)

U E.R. Scrences,

lJniversrt6 de Haute-Normandie,

(Received 16th March 1987; in revised form 14th July 1987)

ABSTRACT The electrochemical oxidation of selenophenes substituted in /3- or P,P’-positions by electron-donatmg groups leads to the formation of conducting polymers. These polymers are formed either by repetitive cyclic voltammetry or by the potential step experiments. The chronoamperometric results show that, for certain derivatives, the film formation process resembles the process observed in the electrodeposition of metals: a first step of nucleation is followed by three-dimensional growth which can be either progressive or instantaneous. These phenomena are governed by the electronic and steric effects of the substituents.

INTRODUCTION

For some years, an increasing number of papers have been devoted to electroactive organic polymers. The growing interest in this research area is explained by the potential applications of these materials [l-6]. Various unsaturated organic molecules have been used for conducting film preparations; they include polycyclic hydrocarbons [7] and heterocyclic compounds [8]. Particular attention has been paid to pyrroles [9,10] and thiophenes [11,12] because of their good stability. Numerous papers describe their physical properties and applications. Due to the structural differences of the materials, however, the results found in the literature are not always consistent. It therefore becomes important to define and control the experimental conditions for polymer synthesis. In this perspective, electrochemical methods are well adapted to the control of film formation.

l

To whom correspondence should be addressed.

OO22-0728/87/$03.50

0 1987 Elsevler Sequoia S.A.

226

We are interested in the electrochemical preparation of polyselenophenes, which, as access to the monomers is difficult, have not yet been extensively studied. Recently, the electrochemical synthesis of a polyselenophene (R, = R, = H) has been published [13]. In a previous paper, we described the synthesis of poly(3-methylselenophene) and poly(3-methoxyselenophene) [14]. We report here the systematic study of the electrochemical polymerization of 3-substituted and 3,4-disubstituted selenophenes: R1

R2

/ H

\

R,

H

Br

Br

Me

Me

Me0

Me0

MeS

Et

R,

H

H

Br

H

Me

H

Me0

H

H

Se

For given experimental conditions (saturated LiClO, in CH,CN), we tried to show the influence of the substituent on the polymer formation. Two methods were used: repetitive cyclic voltammetry, which allows observation of the possible increase in the size of the film from one cycle to the next, and chronoamperometry, a little used technique which, nevertheless, gives interesting information [15-171. This technique visualizes the nucleation possibilities according to the nature of the substituents and also the evolution of the nuclei until they give a film which may or may not be a good conductor. EXPERIMENTAL

All the electrochemical measurements were done in acetonitrile. distilled twice before use [14] with sufficient LiClO, to obtain a saturated solution (0.7-l M). The cell was equipped with a Pt disc (2 mm diameter) and a Ni wire as the working electrode and the counter-electrode, respectively. with Ag/Ag+ (10e2 M) as the reference electrode. Selenophene [18], 3-bromoselenophene [19], 3,4-dibromoselenophene [20], 3methoxyselenophene [21], 3-methylthioselenophene [21], 3-methylselenophene [22], 3,4_dimethylselenophene [22] and 3-ethylselenophene [23] were prepared by published methods. Before use, all these compounds were purified by distillation on a helix packed column. 3,4-Dimethoxyselenophene was obtained as follows: 3,4-Dibromoselenophene (0.024 mol) was added to a sodium methoxide solution in methanol (0.28 mo1/80 ml) in the presence of KI (0.1 g) and CuO (4 g). The mixture was refluxed for 5 days. After solvent elimination, the residue was treated with a dilute acidic aqueous solution and extracted three times with ether. The organic solution was dried, concentrated and the yellow oil obtained was distilled; B. Pt (15 Torr) = 118°C. The product solidified slowly and was crystallized in hexane (F. Pt = 38 o C; yield 72%). 3,4-Dimethoxyselenophene gave a satisfactory elemental analysis and a correct ‘H NMR spectrum (CDCI,; &n, = 3.86 ppm, &n,,n, = 6.56 ppm). All the electrochemical oxidations were performed on l-2 x lo-’ mol l- ’ monomer solutions, previously degassed by bubbling nitrogen through them. Cyclic

221

voltammetric measurements were carried out with a Tacussel PRT 20 potentiostat coupled with a GSTP 3 generator; the curves were visualized on a Gould 4100 oscilloscope, then recorded on a J.J. Lloyd recorder. The chronoamperometric curves were obtained and treated by a PAR 273 potentiostat switched by an Apple 2 E microcomputer. The measurement rate corresponded to one point every 50 ms and the entire experiment lasted 20 s. RESULTS

Cyclic voltammetry In cyclic voltammetry, in the range O-2 V, one or more completely irreversible peaks are observed, depending on the compound and the sweep rate (v) used. Between 0.005 and 0.5 V s-l, the first peak shifts towards positive potentials as v increases and the peak current varies linearly with the square root of the sweep rate. The first stage of oxidation is therefore governed by the diffusion of species in solution. For compound I, whose oxidation potential is very high, there is a single oxidation peak whatever the value of u. For compounds II and IV-VIII, a second oxidation peak (0.1-0.2 V beyond the first) can be observed for v < 0.02 V s-‘. Other very narrow peaks are also visible between 1.6 and 2 V and are accompanied by marked coloration effects in the vicinity of the electrode. These peaks, which indicate irreversible degradation of the deposit formed at lower potentials, disappear gradually as u increases, and at 0.1 V s-r they are completely absent. The second stage of oxidation is therefore no more than a hump on the first oxidation peak (Figs. lb and lc). In the case of selenophene (III), the first two stages of oxidation are very close together and cannot be separated, whatever the value of u (Fig. la). On the other hand, for compound IX, the second stage of oxidation gives a well-formed peak, whatever the value of v, and is 400 mV positive of the first peak (Fig. Id). These differences in the behaviour of substituted and unsubstituted monomers have been observed for pyrroles [15,16] but the second oxidation stage has not yet been clearly explained. Nevertheless, it must be noted that the cyclic voltammetric curve obtained with a monomer-free solution, using a film of poly(3-methylselenophene) as the working electrode, shows among other phenomena a peak whose potential coincides with the second peak discussed above. This therefore is associated with a later oxidation of the film and not with the monomer in solution. Further, if the sweep is stopped just beyond the first peak, a coloured film forms on the electrode. In Table 1, we have grouped together the cyclic voltammetric data obtained at 0.1 V s-l. Comparison of the E,, values of the various monomers (which correspond to the first oxidation peak) shows the importance of the substituent on the electrooxidation of selenophene derivatives:

Fig. 1. Cyclic voltammetry of some selenophene derivatives. 1-2X lo-* mol 1-i m CH,CN + saturated o = 0.1 V ss’; (c) 3-methoxyLiClO,. (a) Selenophene, u = 0.1 V s-‘; (b) 3-methoxyselenophene, selenophene, o = 0.01 V s-l; (d) 3,4_dimethoxyselenophene, u = 0.1 V s-i.

An electron-donating group such as CH, or OCH,, which stabilizes the cation radical formed, diminishes the oxidation potential. In the case of the bromo derivatives (compounds I and II), the presence of the halogen with its withdrawing

TABLE

1

Cyclic voltammetric data of selenophenes in CH,CN+saturated monomer concentration: 1-2X lo-* mol 1-l) Compound

Monomer

E,,a/V I II III IV V VI VII VIII IX

a

3,4-Dibromoselenophene 3-Bromoselenophene Selenophene 3-Methylselenophene 3-Ethylselenophene 3.4-Dimethylselenophene 3-Methoxyselenophene 3-Thiomethylselenophene 3,4-Dimethoxyselenophene

1.82 1.54 1.48 1.4 1.37 1.20 1.18 0.98 0.96

a

LiClO,

(sweep rate:

0.1 V s-i;

Polymer

E,; E,/V

b

0; +1.4 0; +1.35 0; +1.10 -0.6; +l.O -0.3; +0.9 -0.4; + 1.0 d

0.58 0.60 0.80 0.05 = 0.33 0.08 =

0.48 0.55 0.65 - 0.02 0.20 0.01

Epa. E,: anodic and cathodic peak potentials. Ag/lO-* mol 1-i Agf. b E,, Er: negative and positive limits of potential m cychc voltammetry for preparation of polymers ’ Complex peak. d Sweep rate: 0.02 V s-l.

229

inductive effect makes the oxidation more energetic. This is also true if we compare the mono- and disubstituted selenophenes: the latter are oxidized 150-200 mV before or after the monosubstituted derivatives, according to the electronic character of the substituent. These observations corroborate the results observed with substituted thiophenes [24]. In repetitive cyclic voltammetry, the behaviour of the monomers differs according to their oxidation potentials and they can be divided into two categories. (1) Compounds I-III

When the positive limit of the sweep is set very close to the first oxidation peak, its intensity decreases very rapidly: after the third cycle, it represents 5% of the initial value. From the first positive sweep, the electrode is covered by a yellow-brown layer, but during the reverse sweep and the following cycles, no cathodic current is detected. In the case of III, the deposit is a non-conductive film of polyselenophene (see later). However, where the two bromo derivatives are concerned, the nature of the deposit is more difficult to establish since the cation radicals formed during oxidation of the monomer are very reactive and may react on the nucleophilic species present in solution. (2) Compounds IV-IX

In repetitive cyclic voltammetry, the oxidation peak intensity decreases slowly as a consequence of the progressive diminution of the substrate concentration in the diffusion layer. We observe anodic and cathodic peaks with potential values below the monomer oxidation potential (see, for example, Fig. 1 in ref. 14). This new system corresponds to the oxidation and reduction of the polymer. We observe at the same time (except for IX) the formation of a deposit on the electrode. For compounds VII and VIII, a strongly coloured diffusion from the electrode towards the solution shows that the polymerization is not quantitative. For the 3,4-dimethoxyselenophene, which is the more electron-enriched monomer, no layer appears under the experimental conditions described above, whatever the sweep range. With slower sweep rates (0.02 V s-l), the more complex redox system increases steadily and an adhesive polymer is formed on the electrode. We believe that during the negative sweep a low scan rate favours the irreversible precipitation of oligomers of various sizes onto the electrode. The evolution of the formation of the polymer redox system depends on the sweep range. The choice of the positive limit E, is an important factor since it determines homogeneity and consequently the electrochemical response of the polymer. As can be seen in Table 1, the optimal values of E, are very close to the oxidation potential of the monomer. When E, is higher, the electrochemical response of the deposit, studied in a solution free of monomer, decreases rapidly until only the resistive current of the film is obtained. Here also, the phenomenon is more or less marked according to the monomer being studied; the optimal sweep limits must be found

230

231

for each compound when the experimental conditions are changed. We must note, however, that the choice of the negative sweep limit is less important as long as it allows polymer reduction. Chronoamperometry The formation and deposition of polyselenophenes was also studied using potential step experiments, and the i-t curves were recorded over a long time-scale (20 s). Here again, the effect of the substituents is important and different types of curve were obtained: (1) For 3-bromo- and 3,4-dibromoselenophenes, the chronoamperometric curves drawn at different potentials show a decaying current which tends to zero; this confirms the formation of an insulating film on the electrode. (2) With selenophene, the i-t curves (Fig. 2a) exhibit several distinct parts: Initially we observe a sudden current variation corresponding to the double layer (this transient is not shown in the figure). After an induction time (to), the current increases, goes through a maximum, then decreases and tends to a low constant value. When the overpotential is increased, the induction time diminishes, as does the time (t,) corresponding to the maximum current (i,); the maximum current itself also increases. The shapes of these curves are similar to those obtained for the electrocrystallization of metallic compounds [25.26]. The induction period is rarely taken into account in i-t curve analysis [27]. In the case in point, i.e. electropolymerization of heterocycles, we feel that i, corresponds to the formation of oligomers in the solution. This result has recently been established in the case of 3-methyl-thiophene, using ellipsometry [28]. At the end of the induction period (at time to). these oligomers become attached to the electrode, then a nucleation process occurs. Further, the rising part of the current issues from the growth and the overlapping of nuclei on the electrode. The shapes of the early parts of the rising transients were analysed by plotting log i vs. log t but we never obtained any characteristic linearity, whatever the monomer. On the other hand, when the i and t values are corrected for the current i, and the induction period t,, as follows: i,,,, = i - i,

(1)

and t c0l-r=t-tt, the curve log i,,, vs. log t,,,, appears, with a slope of 1.5. So lcorr = Ktk:r

(2) at least for the low t,,,,, as a straight

line

(3)

This relation corresponds to a progressive nucleation with a three-dimensional growth process [29]. When t - co, the current tends to a very low, but not zero, limit, which is explained by the formation of a poorly conducting film. This

232

phenomenon also explains why the polymer does not give an electrochemical response in cyclic voltammetry. It must also be noted that with values higher than 1.26 V/ref, integration of the i-t curves of Fig. 2a leads to an amount of electricity equal to 14 mC/cm2 (at +4%), which corresponds to a coverage I of the electrode of around 6 X lop8 mol cme2. (3) With compounds IV and V, the chronoamperometric curves have the same shape, i.e. we observe an induction period which decreases when the overpotential increases, followed by a rapid increase in the current, corresponding to the birth and growth of the nuclei. Then when the electrode is covered with a continuous layer, the current falls towards a steady-state value. The rising part of the i-t curves (Fig. 2b) is represented by eqn. (3). It is still a progressive three-dimensional nucleation process governed by diffusion of the monomer to the nuclei. This result can be confirmed with the non-dimensional equation of the i-t curve proposed by Scharifker and Hills [30]: I2 =f(

T)

(4)

where I = i/i,

(5)

and T = t/t,,,

(5’)

I2

0

10

2.0

30

4.0

Fig. 3. Non-dimensional plots for (a) instantaneous and (b) progressive three-dimensional nucleation (from ref. 30). Experimental data from Fig. 2b. (0) 1.19 V; (*) 1.20 V; (0) 1.21 V; (0) 1.22 V.

233

The experimental I = (i - i,)/(

data plotted

in Fig. 3 for 3-methylselenophene

i, - i,)

correspond

to (6) (6’)

We obtain a good correlation with the expected result, at least in the first part of the curve. When the potential step is increased, the falling current of the i-t transient, above i,, never becomes independent of the potential. This indicates that for t > t,, the growth of the film has some limitation due to the rate of electron transfer [31]. The change in mechanism is surprising but the i-r curves are absolutely clear and the explanation of the phenomenon must include the modification of the electrode surface during polymerization. For a potential step of about 1.325 V/ref, the charge involved is maximal; at higher overpotentials, the falling transient occurs at a lower level and the charge decreases rapidly. As in repetitive cyclic voltammetry, this trend suggests irreversible degradation of the polymer film when the potential is too high. (4) The behaviour of 3,4_dimethylselenophene is somewhat different (Fig. 2~): the log i,, vs. log tCorr curves, drawn at different potentials (corresponding to the rising of the first cyclic voltammetric peak), give a straight line with a slope of 2. Therefore we can write the equation L,

= K’t,2,,

(7)

i,,,, and t,,, are given by eqns. (1) and (2). The good linearity obtained in Fig. 4 is not an artefact; each line is made up of more than 30 points and the correlation coefficient is 0.999.

T(i-l,)/rA

0

Fig. 4. Linearization (*) 1.055 v.

5

of some of the curves from Fig. 2c. ( * ) 1.01 V; (0)

1.02

234

Equation (7) corresponds either to a progressive two-dimensional nucleation or to instantaneous three-dimensional nucleation [32]. The invariance of i,t, with the potential is a simple criterion for a two-dimensional mechanism [25]. Here, i,t, is thereand (i, - i,)(t, - to) deacrease if the potential increases. The nucleation fore instantaneous, three-dimensional, and the rate-determining step in the growth of the nuclei is electron transfer at their surfaces. However, when t > t,, the current decreases and tends to a constant value which is independent of the potential. The growth of the film is therefore governed by diffusion of the monomer to the surface [33-361. the charge involved during the 20 s of When the potential step increases, electrolysis increases as well, but for potentials higher than 1.3 V, the i-t curves are distorted and there is virtually no further deposit formation on the electrode. (5) For compounds VII-IX, the i-t curves do not show any nucleation phenomena, whatever the potential applied (see, for example, Fig. 2d). The curve shape is coherent, at least in the first part, with a reaction controlled by monomer diffusion, which means that the current varies linearly with t-‘I’. The current becomes constant after a few seconds. As soon as the potential is applied, marked bluish coloration can be observed in the vicinity of the electrode; in this case, the formation of the deposit must occur when the solubility product of the oligomers formed at the electrode is reached. DISCUSSION

It is generally accepted that the electropolymerization of unsaturated polycyclic hydrocarbons and aromatic heterocycles depends on the inductive, mesomeric and steric effects of the substituents [9,37]. According to its stability, the cation radical, derived from the monomer, either polymerizes or leads to other reactions in the solution. So, we can define a potential scale (here, approximately between 0.9 and 1.5 V/ref) in which the polymerization appears (not always quantitatively). Outside this area, the cation radical is either too stable (E -c 0.9 v> or too reactive (E > 1.5 V), leading either to the formation of soluble oligomers diffusing from the electrode towards the solution or to a reaction with the nucleophilic species present in the solution. Moreover, when the oxidation potential is too high, the reactions with the solvent are no longer negligible. A plot of the monomer oxidation potentials against the oxidation potentials of the corresponding polymers gives an approximately straight-line fit (Fig. 5). This shows a similar effect of the substituents on both the monomers and the polymers. Their m-electronic conjugated systems are similar and lead to polymerization with coupling of the a-carbons as expected for five-membered aromatic heterocycles [38]. The linearity does not seem to be as good as that observed for thiophene derivatives [24]. A possible explanation could be found in the weaker aromaticity of the selenophene nucleus [39,40]. Two opposite factors account for this fact: (i) the lower electronegativity of the selenium atom which favours conjugation with the carbon system, and the long C-Se covalent radii (0.117 nm in comparison with

235

4 1.5. > . L E z z

Y

x

1.0.

I

0.5

0 E,,

POIymer

-

1.0

I”

Fig. 5. Peak oxidation potentials of selenophene monomers vs. their respective polymers.

0.104 nm for the C-S bond) which prevents good overlapping of the selenium 4p orbital with the carbon 2p orbital; and (ii) the lack of aromaticity, which gives greater electrophilic reactivity to the selenophene nucleus [41,42]. In spite of the increase in the metallic properties of the heteroatom Se as compared with S, the electrical conductivities of polyselenophenes are significantly lower than those of polythiophenes. For example, the oxidized poly(3-CH, selenophene) and poly(3,4-diCH, selenophene) prepared at a fixed potential under the experimental conditions given here have conductivity values of around 9 x lo-’ and 6 X lop9 E’ cm-‘, respectively. On the other hand, very good stability of polyselenophene derivatives during the oxidation and reduction processes has been reported [43]; chronocoulometric experiments performed by repetitive potential steps from 0 to 0.5 V and the reverse, applied to a film of poly(3-methylselenophene) show that after 2 x lo5 cycles, the charge involved during the oxidation still represents 85% of the initial charge. CONCLUSION

Repetitive cyclic voltammetry and chronoamperometry applied to the synthesis of substituted polyselenophenes provide interesting information. The first method shows that the formation of an electroactive polymer on the electrode needs the presence of electron-donating groups on the heterocycle P-carbons. However, this method does not allow us, for example in the case of selenophene, to explain the absence of an electrochemical response from the formed layer. The polymeric nature of this layer is shown by the chronoamperometric study. The i vs. t curves allow us

236

to visualize in situ the birth and the three-dimensional growth of the nuclei on the electrode When the nucleation is progressive (compounds III, IV and v), the film is built up of chains of various lengths; this partially explains the very wide polymer redox peaks commonly observed in cyclic voltammetry. Indeed, when the substituents lead to very good heterocyclic stability, the i-t curves indicate formation of certain polymers (or oligomers) in the solution, followed by precipitation on the electrode. A study in electronic microscopy of the polyselenophenes prepared in this work is currently under way and will provide complementary information to that supplied by the electrochemical methods. ACKNOWLEDGEMENT

The authors wish to thank ity measurements.

M. Fauvarque

(C.G.E.

Marcoussis)

for the conductiv-

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

K. Kaneto, K. Yoshino and Y. Inuishi, Jpn. J. Appl. Phys.. 22 (1983) L412. M.A. Druy and R.J. Seymour, J. Phys. (Paris) Colloq., 44 (1983) C3-595. G. Tourillon and F. Garmer, J. Phys. Chem., 88 (1984) 5281. M.A. De Paoli, R.J. Waltman, A.F. Diaz and J. Bargon, J. Chem. Sot., Chem. Commun., (1984) 1015. K. Kaneto, S. Takeda and K. Yoshino, Jpn. J. Appl. Phys., 24 (1985) L553. M. Aizawa, T. Yamada, H. Shinohara, K. Akagi and H. Shirakawa. J. Chem. Sot., Chem. Commun., (1986) 1315. R.J. Wahman and J. Bargon, J. Electroanal. Chem., 194 (1985) 49. R.J. Waltman, A.F. Diaz and J. Bargon. J. Phys. Chem., 88 (1984) 4343, E.M. Genies. G. Bidan and A.F. Diaz, J. Electroanal. Chem., 149 (1983) 101. A.F. Diaz. J. CastiBo, K.K. Kanazawa. J.A. Logan, M. Salmon and 0. Fyardo. J. Electroanal. Chem., 133 (1982) 233. G. Tourillon and F. Gamier, J. Electroanal. Chem., 135 (1982) 173. R.J. Waltman. J. Bargon and A.F. Diaz, J. Phys. Chem., 87 (1983) 1459. K. Yoshino, Y. Kohno, T. Shiraishi, K. Kaneto, S. Inoue and K. Tsukagosm, Synth. Met., 10 (1985) 319. G. Dian, G. Barbey and B. Decroix, Synth. Met., 13 (1986) 281. S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem., 177 (1984) 229. S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena and D. PIetcher, J. Electroanal. Chem.. 177 (1984) 245. A.J. Downard and D. Pletcher, J. Electroanal Chem., 206 (1986) 139. S. Mohmand, J. Bargon and R.J. Wahman, J. Org. Chem., 48 (1983) 3544. Yu.K. Yur’ev, N.K. Sadovaya and E.A. Grekova, Zh. Obshch. Khim., 34 (1964) 847. C. Pauhnier, J. Morel and P. Pastour, Bull. Sot. Chim. Fr., (1969) 2511. J. Morel, C. Paulmier, M. Garreau and G. Martin, Bull. Sot. Chim. Fr., (1971) 4497. Yu.K. Yur’ev and L.I. Khmel’nitsku, Dokl. Akad. Nauk SSSR, 94 (1954) 265. S. Gronowitz, A. Konar and A.B. HBmfeIdt, Chem. Ser., 10 (1976) 159. G. Tourillon and F. Gamier, J. Electroanal Chem., 161 (1984) 51. H.R. Tlursk and J.A. Harrison, A. Guide to the Study of Electrode Kinetics, Academic Press, London, 1972, Ch. 3.

231 26 R. Greef. R. Peat, L.M. Peter, D. Pletcher and J. Robinson, Instrumental Methods m Electrochemistry, Ellis Horwood, Chichester, 1985, Ch. 9. 27 M.J. Lain and D. Pletcher, Electrochim. Acta, 32 (1987) 99. 28 P. Lang, F. Chao, M. Costa, E. Lheritier and F. Gamier, Commumcation at JoumCes d’Electrochunie 1987, DiJon, France, l-4 June 1987. 29 G.J. Hills, D.J. Schiffrin and J. Thompson, Electrochim. Acta, 19 (1974) 657. 30 B. Scharifker and G. Hills, Electrochim. Acta, 28 (1983) 879. 31 G. Gunawardena, G. Hills, I. Montenegro and B. Scharifker, J. Electroanal. Chem., 138 (1982) 225. 32 J.A. Harrison and H.R. Thirsk m A.J. Bard (Ed.), Electroanalytical Chemtstry, Vol. 5, Marcel Dekker, New York, 1971, Ch. 2. 33 M.Y. Abyaneh, Electrochrm. Acta, 27 (1982) 1329. 34 M Fleischmann and H.R. Thirsk m P. Delahay and C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 3, Interscience, New York, 1963, p. 123. 35 R.D. Armstrong, M. Fleischmann and H.R. Thirsk. J. Electroanal. Chem., 11 (1966) 208. 36 M.Y. Abyaneh, J. Electroanal. Chem., 209 (1986) 1. 37 R.J. Waltman and J. Bargon, Can. J. Chem., 64 (1986) 76. 38 M. Renson in S. Patai and 2. Rappoport (Eds.), The Chemistry of Orgamc Selenium and Tellurium Compounds, Vol. 1, Wiley, New York, 1986, Ch. 13. 39 R.H. Findlay. J. Chem. Sot., Faraday Trans. 2, 71 (1975) 1397. 40 F. Frmguelli, G. Marino, A. Taticchi and G. Grandolini, J. Chem. Sot., Perkin Trans. 2, (1974) 332. 41 P. Linda and G. Marina, J. Chem. Sot. B, (1970) 43. 42 G.R. Newkome and W.W. Paudler, Contemporary Heterocychc Chemistry, Wiley, New York, 1982, p. 94. 43 N. Merlet, G. Dian, G. Barbey, F. Outurqum and C. Paulmier, Synth. Met., submitted.